To create memory cells of new generation, epitaxial layers of GeSbTe (GST) material, with high crystalline perfection, and multilayered crystalline structures based on GeTe/SbTe superlattices grown on Si-wafers are of interest. This initiates the study of these and alike materials generation patterns, including with the involvement of molecular beam epitaxy. In this work, the structure of 13 nm thick layers of GST material, used to create phase-change memory cells was studied. These layers were grown on an Sb-passivated Si(111) substrate by molecular beam epitaxy. Research studies were carried out by transmission electron microscopy and electron diffraction analysis. Using high-resolution images of cross-sectional samples and diffraction patterns from planar thin foils, it was revealed that the layer consists of crystalline grains, mostly hexagonal, and in some local regions of the vacancy ordered cubic GST phase with the GST(0001) and GST(111) planes parallel to the Si(111). Based on an analysis of the moiré pattern appearing in bright-field electron microscopy images, it was found that the misorientation of the grains of the epitaxial layer around the Si(111) direction varies from 0 to 13.5º, and nearly 26 % of the surface area is almost non-rotated grains. Grains rotates within the angles from 0.2 to 2º occupy about 34 % of the layer surface area, from 2 to 8º occupy about 33 %, and the fraction of the area of grains rotated by more than 8º is close to 7 %. It has been found that as the rotation angle of the GST grains relative to the substrate increased, their average lateral size decreased from about 150 nm for non-rotated grains to 80 nm for grains rotated at an angle of more than 8º, and the average value of the rotation angle was approximately 2.6º. The data obtained on the grain structure of the epitaxial layer indicate that the relaxation of the misfit stresses of the crystal lattices of silicon and the GST material is provided both by the rotation of grains and, apparently, by the formation of misfit dislocations.
-
Bibliography link:
Электронно-микроскопические исследования структуры тонкого эпитаксиального слоя Ge2Sb2Te5, выращенного на подложке Si(111) / Ю.С. Зайцева, Н.И. Боргардт, А.С. Приходько и др. // Изв. вузов. Электроника. 2021. Т. 26. № 3-4. С. 214–225. DOI: https://doi.org/10.24151/1561-5405-2021-26-3-4-214-225
1. Redaelli A. Phase change memory: Device physics, reliability and applications. Cham: Springer International Publishing AG, 2018. XVIII, 330 p. DOI: https://doi.org/10.1007/978-3-319-69053-7
2. Lotnyk A., Behrens M., Rauschenbach B. Phase change thin films for non-volatile memory applications // Nanoscale Advances. 2019. Vol. 1. No. 10. P. 3836–3857. DOI: https://doi.org/10.1039/C9NA00366E
3. Interfacial phase-change memory / R.E. Simpson, P. Fons, A.V. Kolobov et al. // Nature Nanotechnology. 2011. Vol. 6. No. 8. P. 501–505. DOI: https://doi.org/10.1038/nnano.2011.96
4. Boschker J.E., Calarco R. Growth of crystalline phase change materials by physical deposition methods // Advances in Physics: X. 2017. Vol. 2. No. 3. P. 675–694. DOI: https://doi.org/10.1080/23746149.2017.1346483
5. In situ observations of the reversible vacancy ordering process in Van der Waals-bonded Ge-Sb-Te thin films and GeTe-Sb2Te3 superlattices / A. Lotnyk, T. Dankwort, I. Hilmi et al. // Nanoscale. 2019. Vol. 11. P. 10838–10845. DOI: https://doi.org/10.1039/C9NR02112D
6. Elswijk H.B., Dijkkamp D., van Loenen E.J. Geometric and electronic structure of Sb on Si(111) by scanning tunneling microscopy // Physical Review B. 1991. Vol. 44. Issue 8. P. 3802–3809. DOI: https://doi.org/10.1103/PhysRevB.44.3802
7. Tailoring the epitaxy of Sb2Te3 and GeTe thin films using surface passivation / J. Momand, J.E. Boschker, R. Wang et al. // CrystEngComm. 2018. Vol. 20. Iss. 3. P. 340–347. DOI: https://doi.org/10.1039/C7CE01825H
8. Toward truly single crystalline GeTe films: the relevance of the substrate surface / R. Wang, J.E. Boschker, E. Bruyer et al. // Journal of Physical Chemistry C. 2014. Vol. 118. No. 51. P. 29724–29730. DOI: https://doi.org/10.1021/jp507183f
9. Surface reconstruction-induced coincidence lattice formation between two-dimensionally bonded materials and a three-dimensionally bonded substrate / J.E. Boschker, J. Momand, V. Bragaglia et al. // Nano Letters. 2014. Vol. 14. No. 6. P. 3534–3538. DOI: https://doi.org/10.1021/nl5011492
10. Epitaxial Ge2Sb2Te5 films on Si(111) prepared by pulsed laser deposition / I. Hilmi, E. Thelnader, P. Schumacher et al. // Thin Solid Films. 2016. Vol. 619. P. 81–85. DOI: https://doi.org/10.1016/j.tsf.2016.10.028
11. Nakaoka T., Satoh H., Honjo S., Takeuchi H. First-sharp diffraction peaks in amorphous GeTe and Ge2Sb2Te5 films prepared by vacuum-thermal deposition // AIP Advances. 2012. Vol. 2. P. 042189. DOI: https://doi.org/10.1063/1.4773329
12. Pulsed laser deposited GeTe-rich GeTe-Sb2Te3 thin films / M. Bouška, S. Pechev, Q. Simon et al. // Scientific Reports. 2016. Vol. 6. No. 1. P. 26552. DOI: https://doi.org/10.1038/srep26552
13. Author correction: Modulation of Van der Waals and classical epitaxy induced by strain at the Si step edges in GeSbTe alloys / E. Zallo, S. Cecchi, J.E. Boschker et al. // Scientific Reports. 2018. Vol. 8. No. 1. P. 1–2.
14. Research update: Van-der-Waals epitaxy of layered chalcogenide Sb2Te3 thin films grown by pulsed laser deposition / I. Hilmi, A. Lotnyk, J.W. Gerlach et al. // APL Materials. 2017. Vol. 5. Iss. 5. P. 050701. DOI: https://doi.org/10.1063/1.4983403
15. Sosso G.C., Caravati S., Mazzarello R., Bernasconi M. Raman spectra of cubic and amorphous Ge2Sb2Te5 from first principle // Physical Review B. 2011. Vol. 83. Iss. 13. P. 134201. DOI: https://doi.org/10.1103/PhysRevB.83.134201
16. Evolution of low-frequency vibrational modes in ultrathin GeSbTe films / E. Zallo, D. Dragoni, Y. Zaytseva et al. // Physica Status Solidi (RRL). 2021. Vol. 15. Iss. 3. P. 2000434. DOI: https://doi.org/10.1002/pssr.202170014
17. Epitaxial formation of cubic and trigonal Ge-Sb-Te thin films with heterogeneous vacancy structures / I. Hilmi, A. Lotnyk, J.W. Gerlach et al. // Materials and Design. 2017. Vol. 115. P. 138–146. DOI: https://doi.org/10.1016/j.matdes.2016.11.003
18. Surface energy driven cubic-to-hexagonal grain growth of Ge2Sb2Te5 thin film / Y. Zheng, Y. Cheng, R. Huang et al. // Scientific Reports. 2017. Vol. 7. No. 1. P. 5915. DOI: https://doi.org/10.1038/s41598-017-06426-2
19. Robust topological surface states in Sb2Te3 layers as seen from the weak antilocalization effect / Y. Takagaki, A. Giussani, K. Perumal et al. // Physical Review B. 2012. Vol. 86. No. 12. P. 125137. DOI: https://doi.org/10.1103/PhysRevB.86.125137
20. Andrieu S. Sb adsorption on Si<111> analyzed by ellipsometry and reflection high-energy electron diffraction: Consequences for Sb doping in Si molecular-beam epitaxy // Journal of Applied Physics. 1991. Vol. 69. No. 3. P. 1366–1370. DOI: https://doi.org/10.1063/1.347274
21. Mayer J., Giannuzzi L.A., Kamino T., Michael J. TEM sample preparation and FIB-induced damage // MRS Bulletin. 2007. Vol. 32. Iss. 5. P. 400–407. DOI: https://doi.org/10.1557/mrs2007.63
22. Role of vacancies in metal–insulator transitions of crystalline phase-change materials / W. Zhang, A. Thiess, P. Zalden et al. // Nature Materials. 2012. Vol. 11. P. 952–956. DOI: https://doi.org/10.1038/nmat3456
23. STEM_CELL // (Quantum) e-Optics and TEM GROUP: [Web] / CNRNANO. URL: http://tem-s3.nano.cnr.it/?page_id=2 (accessed: 08.04.2020).
24. Grillo V., Rotunno E. STEM_CELL: A software tool for electron microscopy: Part 1: Simulations // Ultramicroscopy. 2013. Vol. 125. P. 97–111. DOI: https://doi.org/10.1016/j.ultramic.2012.10.016
25. Williams D.B., Carter C.B. Transmission electron microscopy. A textbook for materials science. New York: Springer US, 2009. LXII, 775 p. DOI: https://doi.org/10.1007/978-0-387-76501-3